Conventionally, powder magnetic cores have been used as magnetic cores provided in electromagnetic devices such as motors, generators and reactors. In general, the powder magnetic cores of this kind are manufactured by compacting a soft magnetic material (powder) containing iron as the main component on which a thin insulating layer is formed by a phosphate treatment or the like for the purpose of improving the insulation property and increasing the density of magnetic flux. After the compaction, a heat treatment (annealing) is performed in order to release compression strain caused during the compaction and reduce iron loss (core loss).
Powder magnetic cores that are driven in an alternating magnetic field are generally required to exhibit a small core loss. It is effective to increase the temperature in the heat treatment in order to reduce the core loss. However, in a powder magnetic core fabricated using iron powder which has been subjected to an insulating treatment such as a phosphate treatment, since the heat resistance of a phosphoric acid coating is low, the core resistance is likely to be reduced by a heat treatment of 500° C. or higher and eddy-current loss increases, and as a result, the core loss cannot be sufficiently reduced.
In addition, in motors and generators, the powder magnetic core is required to exhibit a high magnetic flux density, and in order to achieve a high magnetic flux density, it is desired that the density of the powder magnetic core be high as long as the same material is used. However, if a compacting pressure is increased in order to increase the density of the core, force acting on particles and on the interface between the particles increases, and thus the insulating layers are destroyed or peeled off, which results in a reduction in an electrical resistivity. As described above, it has been difficult to achieve compatibility between a high magnetic flux density and a high electrical resistivity.
In order to solve this problem, for example, Patent Document 1 discloses a powder magnetic core in which particulate fluoride and a peelable body constituted by iron oxide both exist in an insulating layer. Also, Patent Document 2 discloses a powder magnetic core in which coatings made of an iron phosphate compound and an aluminum phosphate compound.
[Patent Document 1] Japanese laid-open publication No. 2010-123699
[Patent Document 2] Japanese laid-open publication No. 2006-128663
However, in the technique disclosed in Patent Document 1, although the density of a compact is increased by increasing the compacting pressure, the technique has a drawback in that the electrical resistivity cannot be increased. In the technique disclosed in Patent Document 2, the density of magnetic flux of the resulting powder magnetic core is lowered greatly, and thus the technique has a drawback in that the compatibility between a high electrical resistivity and a high magnetic flux density cannot be achieved.